Metal oxide catalyst and use thereof in chemical reactions

ABSTRACT

A method for conducting a chemical reaction with a catalyst composed of metal oxide particles among which are uniformly incorporated, in order to reduce the operating temperature of the catalyst, palladium particles.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 08/164,413, filed Dec. 10,1993, now U.S. Pat. No. 5,478,528; and a continuation-in-part of Ser.No. 08/283,569, filed Aug. 1, 1994, now U.S. Pat. No. 5,593,935.

This invention relates to a metal oxide catalyst, its production, amethod of conducting a chemical reaction employing it, and a hazardousgas sensor containing a particular example of it.

It has been discovered how a metal oxide catalyst can be operated at alower temperature.

The invention provides a catalyst composed of metal oxide particlesamong which are uniformly incorporated, in order to reduce the operatingtemperature of the catalyst, palladium particles.

The invention provides also a process for preparing the catalyst, whichprocess comprises co-precipitating the metal oxide particles and thepalladium particles.

The invention also provides a method of conducting a chemical reactionemploying the catalyst.

The invention provides also a sensor of hazardous gas at ambienttemperature, which sensor comprises means to allow gas to contact thepresent catalyst wherein the metal oxide comprises iron(III) oxide andmeans to indicate the rise in temperature of the catalyst if hazardousgas is present.

The invention provides, in a catalyst composed of metal oxide particles,the improvement which comprises incorporating palladium particlesuniformly among the metal oxide particles to reduce the operatingtemperature of the catalyst.

The invention similarly provides, in a method of conducting a chemicalreaction employing a catalyst composed of metal oxide particles, theimprovement comprising incorporating palladium particles uniformly amongthe metal oxide particles to reduce the temperature of the reaction.

It has been discovered that the operating temperature of a catalystcomposed of metal oxide particles can be reduced by incorporatingpalladium particles uniformly among them. In this structure, there ishigh and even interaction between the two sets of particles. Thiscontrasts, for instance, with metal oxide particles whose surface hassimply been impregnated with the palladium.

As is conventional in this art, references herein to a catalystcomponent being palladium embrace the possibility of some or all of itbeing in the form of the oxide.

The present catalyst usually contains 0.1 to 25%, preferably 1 to 20%,by mass of the palladium particles based on the total mass of thepalladium particles and the metal oxide particles.

Additives can be incorporated into the catalyst to convey advantageousproperties or avoid disadvantageous properties. The additives can beconventional. The additives can be for instance antimony oxide, oralkali metal ions to improve selectivity in partial oxidation reactions.Additives can be present for instance in amounts of 0.1-50% of the totalmass of the catalyst. The lower amounts are appropriate for additivessuch as alkali metal ions, and the higher for additives such as antimonyoxide. The usual antimony oxide can be employed, generally antimony(V)oxide or that known as diantimony tetroxide.

The lowering of the operating temperature of catalysts composed of metaloxide particles is a general phenomenon (providing, of course,thermodynamic considerations do not render this impossible). It is ofparticular interest where the metal oxide comprises reducible metaloxide, ie the metal oxide is capable of reduction to another oxide ofthe metal.

The metal oxide may be any which is known to be catalytically active. Itcan be the oxide of a single metal (for instance iron(III) oxide,cerium(IV) oxide, niobia, magnesium oxide, vanadia or antimony oxide) ora mixture thereof (for instance a mixture of cerium(IV) oxide andantimony oxide or a mixture of vanadia and magnesium oxide), a mixedmetal oxide (for instance bismuth molybdate) or a mixture thereof, or asolid solution of one metal oxide in another (which is not necessarilystoichiometric) or a mixture thereof, or a mixture of more than one ofthese types. The usual antimony oxide can be employed, generallyantimony(V) oxide or that known as diantimony tetroxide.

The particle diameter of the catalyst, as measured by sieving, isusually less than 150 microns.

Preferably the catalyst is such that in one or more of the followingreactions, it reduces the temperature at which 9% mol conversion occursby at least 50° C., preferably at least 100° C., compared to that in thecase of the catalyst without the palladium:

(A) the conversion of but-1-ene to butadiene using a gas mixture ofbut-1-ene and air (1/6 by volume) at a flow-rate of 100 cm⁻³ min⁻¹ per gof the catalyst;

(B) the conversion of carbon monoxide to carbon dioxide using a gasmixture of carbon monoxide and air (1/99 by volume) at a flow-rate of2000 cm³ min⁻¹ per g of the catalyst;

(C) the conversion of carbon monoxide and steam to carbon dioxide andhydrogen using a gas mixture of by volume 0.1% carbon monoxide, 10%steam and the balance nitrogen, at a flow-rate of 1250 cm³ min⁻¹ per gof the catalyst; and

(D) the conversion of isobutane to isobutene using a gas mixture ofisobutane and air (1/2 by volume) at a flow-rate of 100 cm³ min⁻¹ per gof the catalyst.

The catalyst is preferably that preparable by co-precipitation of themetal oxide particles and the palladium particles. Co-precipitation is avery effective method of incorporating the palladium particles into themetal oxide particles, but an alternative preparation which gives thesame result would suffice. The co-precipitation can be carried out in amanner known generally per se, conveniently at ambient temperature. Theco-precipitation is preferably carried out so that it occurs in acontrolled rather than a sudden manner.

It will be understood that the co-pecipitation may produce a precipitatewhich does not have the metal oxide present as such, but in a form, suchas an hydroxide, which is then converted to the metal oxide. Theconversion can be accomplished for instance by heating, for example at50°-500° C.

The chemical reaction in which the present catalyst is employed can beany in which a catalyst without the palladium can be employed. Where theunmodified metal oxide acts as a catalyst in several differentreactions, each reaction may be made to occur at a lower operatingtemperature by means of the present invention. The present reaction isusually conducted at a temperature below 700° C., usually at atemperature within the range of ambient temperature up to 700° C.

By being able to operate a reaction at a lower temperature, a saving ofenergy can be achieved and the catalyst will tend to last longer. Inaddition, reactions can now be conducted at ambient temperature whichpreviously required heating the catalyst. In a particular embodiment,the present reaction is conducted at ambient temperature. The lowtemperature activity of the present catalyst is sustained rather thanbeing transitory. It usually lasts, without regeneration, for at least 5hours, preferably at least 100 hours, particularly when the catalyst isprepared by co-precipitation.

It is another advantage of the present invention that the catalyst canbe employed without prior calcination to activate it. It can simply bewashed and dried (at no more than 130° C.) and then used for lowtemperature catalysis. However, calcination may be desirable to ensurephysical stability.

The present chemical reaction is usually oxidation. A preferred reactionis oxidative dehydrogenation, particularly of alkene, especially ofbut-1-ene to butadiene.

A preferred metal oxide comprises (i.e. consists of or includes) bismuthmolybdate. Another preferred metal oxide comprises iron(III) oxide. Afurther preferred metal oxide comprises cerium(IV) oxide optionally inadmixture with antimony oxide. Catalysts containing these metal oxidescan catalyse the reactions normally associated with the unmodified metaloxide from which they are derived.

The catalyst wherein the metal oxide comprises iron(III) oxide is ableto oxidise carbon monoxide to carbon dioxide, even at ambienttemperature. It is not deactivated by the presence of water vapour.Hence, in an advantageous embodiment it is employed to oxidise carbonmonoxide to carbon dioxide in the presence of 0 to 15% water vapour. Gasmixtures referred to in this specification are by volume unlessotherwise indicated. This catalyst will also tolerate gas mixturescontaining nitrogen oxides and/or sulphur compounds. Hence, inadvantageous embodiments it is employed to oxidise carbon monoxide tocarbon dioxide in the presence of 0 to 0.2% nitrogen oxides and/or 0 to0.005% sulphur compounds. It has been found that the rate of COconversion by this catalyst is linearly dependent on CO concentration,over a range of up to 5% or more by volume in gas such as air. Theenergy released during reaction is, therefore, proportional to the COconcentration in the gas, making this catalyst particularly suitable tobe used for CO sensing. It may also be used to sense, in the absence ofCO, other hazardous gases, usually a reducing gas, such as hydrogen oralkene, for instance but-1-ene. In a preferred embodiment, the sensingis at ambient temperature, hence without the need for pellistertechnology.

The sensor can be of type known in itself. Usually the sensor comprisesmeans to allow gas to contact the catalyst wherein the metal oxidecomprises iron(III) oxide and means to indicate the rise in temperatureof the catalyst if hazardous gas, especially CO, is present. Aparticular advantage of sensing at ambient temperature is that thecatalyst does not have to be kept at a raised temperature, so avoidingthe risk of igniting combustible gas. The present catalyst wherein themetal oxide comprises iron(III) oxide is useful for monitoring theperformance of catalytic material for oxidising carbon monoxide tocarbon dioxide, for instance in an engine exhaust. Accordingly, in apreferred embodiment, this catalyst is downstream of catalytic materialin the exhaust system of an engine, the catalytic material being foroxidising carbon monoxide to carbon dioxide and the sensor monitoringthe performance of the catalytic material in this oxidation. Thiscatalyst is particularly suited to automobile applications, and canthere be used in on-board diagnostics, such as monitoring theperformance of catalytic material for treating the automotive exhaust tocombat air pollution. Hence, in a preferred embodiment, the engine is aninternal combustion engine in a vehicle and the monitoring indicateswhen the performance of the catalytic material (for instance a three-waycatalyst) falls below a set level. Compared to a prior art lowtemperature CO oxidation catalyst (Au/Fe₂ O₃, see page 33 of "SuccessfulDesign of Catalysts", edited by T. Inui, published by Elsevier,Amsterdam, 1988), the present catalyst wherein the metal oxide compisesiron(III) oxide has the advantages of (a) lower material costs, (b)lower light-off temperature at the same loading of precious metals, and(c) greater resistance to deactivation.

In the presence of CO and H₂ O, the present catalyst wherein the metaloxide comprises iron(III) oxide functions as a water-gas shift catalyst.This activity begins about 100° C. The catalyst activates in situ, sopre-reduction is not necessary. It can be used in the water-gas shiftreaction at, for instance, 100°-200° C.

The present catalysts, particularly those wherein the metal oxidecomprises iron(III) oxide or bismuth molybdate, are active in theoxidative dehydrogenation of alkene. The alkene is usually acyclic, andcan be straight or branched chain. It generally is of 2-6 carbon atoms.The oxidative dehydrogenation of but-1-ene to butadiene is of particularimportance, and the present catalyst effects this at much lowertemperatures than expected. This reaction with the present catalystwherein the metal oxide comprises iron(III) oxide will begin even if theinitial temperature of the reactor is below 100° C.; once the reactionhas started, for instance after several minutes, it can becomeself-sustaining without the further supply of heat. In a preferredembodiment, the oxidative dehydrogenation of but-1-ene to butadiene isconducted employing as catalyst the present catalyst wherein the metaloxide comprises iron(III) oxide at a temperature below 200° C., forinstance at a temperature between 80 ° and 200° C.

When the present catalyst wherein the metal oxide comprises iron(III)oxide or bismuth molybdate is employed in the oxidative dehydrogenationof but-1-ene to butadiene, the selectivity to butadiene improves withtime on line.

The operating temperature of the present catalyst wherein the metaloxide comprises iron(III) oxide for (a) CO oxidation, (b) water-gasshift, and (c) oxidative dehydrogenation is about 250° C. below that forthe corresponding catalyst without the palladium. Similarly, other ofthe present catalysts, for instance that wherein the metal oxidecomprises bismuth molybdate, have a substantially lower minimumoperating temperature than the corresponding catalyst without thepalladium.

When the metal oxide comprises bismuth molybdate, the oxidativedehydrogenation of but-1-ene to butadiene is preferably conducted at atemperature of 200° to 300° C. This is some 100°-150° C. lower thanconventional selective oxidation catalysts require.

The present catalysts, particularly those wherein the metal oxidecomprises cerium(IV) oxide, optionally with antimony tetroxideincorporated by mixing into the catalyst, are active in thedehydrogenation, oxidative or not, of alkane of at least 2 carbon atomsto alkene. The alkane is usually acyclic, and can be straight orbranched chain. It generally is of 2-6 carbon atoms. A preferred suchreaction is the dehydrogenation of isobutane to isobutene.

When the metal oxide comprises cerium(IV) oxide, and as additivediantimony tetroxide is incorporated by mixing into the catalyst, agiven yield in the dehydrogenation of isobutane to isobutene occursabout 100° C. lower than in the case of the corresponding catalystwithout the palladium. Isobutene is useful for instance formanufacturing the petrol additive methyl t-butyl ether.

Analysis indicates that the present catalyst is usually at leastpartially amorphous, with metal ions and Pd²⁺ ions on the surface.Chemical analysis of the present dry precipitate wherein the metal oxideis iron(III) oxide shows the presence of Fe³⁺ and Pd²⁺ ions on thesurface of a predominantly amorphous bulk. A high degree of interactionbetween the Pd and Fe phases is inferred from temperature-programmedreduction, which shows substantial shifts (to lower temperatures) of thepeaks associated with the reduction of Fe³⁺. Chemical analysis of thepresent dry precipitate wherein the metal oxide is bismuth molybdateshows a mixture of α-bismuth molybdate (Bi₂ Mo₃ O₁₂ ; monoclinic) andβ-bismuth molybdate (Bi₂ Mo₂ O₉ ; monoclinic), with there being noevidence of a crystalline Pd-phase. A conventional Bi--Mo--O catalyst(used for comparison in Comparative Example 2 hereafter) is also amixture of two bismuth molybdates, but these are the β and γ (Bi₂ MoO₆ ;orthorhombic) allotropes. Surface analysis of the present dryprecipitate wherein the metal oxide is cerium(IV) oxide shows thatessentially all the palladium is present as Pd²⁺, and all the cerium asCe⁴⁺.

The present catalyst can be employed as the sole catalyst or togetherwith another catalyst, usually comprising one or more of Pt, Pd, Rh andbase metal oxide. The present catalyst can be formulated in the usualway to catalyse chemical reactions. When it is employed as solecatalyst, it generally does not need to be dispersed on a separate highsurface area carrier. When it is employed together with anothercatalyst, a high surface area carrier is often useful to carry bothcatalysts. For instance, the present catalyst can be dispersed on thecarrier and, either before or usually afterwards, the other catalyst canbe dispersed on the carrier, for instance in the usual way byimpregnating with a precursor and calcining to convert the precursor tothe other catalyst. The present catalyst itself preferably has aBrunauer Emmett Teller surface area of at least 50, especially at least100, m² g⁻¹. The catalyst can be employed in the form of pellets. It canbe employed on a support, preferably a monolith, for instance ahoneycomb monolith. The monolith can be metal, in which case it canreadily be heated, for instance by passing an electrical current throughthe metal. Alternatively the monolith can be ceramic. A separate heaterof gas can be utilised up-stream of the catalyst to heat gas tofacilitate its reaction on the catalyst.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 of is a graph showing the effect of varying the CO concentrationon CO conversion and the size of the exotherm generated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is illustrated by the following Examples.

EXAMPLE 1

A Pd--Fe--O catalyst with a nominal Pd loading of 20% (by mass) wasprepared using crystalline Fe(NO₃)₃.9H₂ O (52.2 g), aqueous Pd(NO₃)₃(28.0 g of solution≡2.30 g Pd) and solid Na₂ CO₃ (30.1 g). The twonitrates were added to demineralised water (1 dm³) in a large beaker (2dm³) fitted with a pH probe and a mechanical stirrer. After dissolvingthe Na₂ CO₃ (in 250 cm³ demineralised water), the aqueous precipitantwas added to the stirred Pd/Fe solution at a rate of 2 cm³ min⁻¹ (usinga peristaltic pump).

The Pd/Fe solution was initially orange and had a pH of 1.3, but turneddarker as the precipitant was added. The precipitate began to form at apH of 2.5, and was accompanied by the evolution of carbon dioxide. Asthe end-point was approached, the suspension became very dark andviscous, and the pH changed more rapidly. At this point, the rate ofaddition of the precipitant was decreased (to 1 cm³ min⁻¹), and thenstopped when a pH of 8.5 was reached. The brown gelatinous precipitatewas isolated (by filtration), washed thoroughly and dried (110° C.; 16h). The dry material (12 g) was found to contain 19% Pd (and <0.01% Na)by mass.

A sample (0.20 g) of the dry material (sieved to a particle diameter<150 μm) was tested under a gas mixture of CO/air at a flow rate of 400cm³ min⁻¹. Gas mixtures in this specification are by volume unlessotherwise indicated. The initial temperature of the catalyst bed was 50°C. FIG. 1 of the accompanying drawings shows the effect of varying theCO concentration on:

(i) CO conversion (the amount of CO converted as % of total gas flow),

(ii) the size of the exotherm generated.

(i) shows a linear dependence on CO concentration (over the range 0-5%),indicating that Pd--Fe--O could be used for CO-sensing. (ii) shows aparticularly convenient way of doing this (over the range 0-4%).

EXAMPLE 2

Several Pd--Fe--O catalysts with different Pd loadings were prepared bythe controlled addition of aqueous Na₂ CO₃ to a mixed solution ofiron(III) nitrate and sodium tetrachloropalladite; the addition of theprecipitant was stopped when the pH reached 8.5. An analogous series ofprior art Au/Fe₂ O₃ (page 33 of "Successful Design of Catalysts", editedby T. Inui, published by Elsevier, Amsterdamn, 1988) catalysts wasprepared by substituting tetrachloroauric acid for the Pd-precursor.

The dry materials were tested under a CO/air (1/99) gas mixture at a gashourly space velocity of 33000 h⁻¹. The conversion of CO was measured asa function of gas inlet temperature. Values for T₅₀ (temperature atwhich CO conversion reaches 50%) were recorded and averaged for severaltemperature programmed tests. For each loading, the T₅₀ value for thePd-containing catalyst was lower than for the Au-containing analogue(Table 1).

                  TABLE 1    ______________________________________    Light-off (T.sub.50) temperatures for the oxidation of CO over Pd--Fe--O    and Au/Fe.sub.2 O.sub.3.    Precious Metal  T.sub.50 /°C.    loading/mass %  Pd--Fe--O Au/Fe.sub.2 O.sub.3    ______________________________________    0               275       275    1               75        230    2               66        120    4               23         86    8               60         81    ______________________________________

When the best Au-containing catalyst (8% Au/Fe₂ O₃) was exposed to thegas mixture at 140° C., the initial CO conversion was 100%, but declinedto <20% over a period of 140 hours. The Pd--Fe--O catalysts did not showthe same deactivation. Their activity remained at 100% during the first10 hours, and was still >80% after 140 hours.

EXAMPLE 3

The ability of 20% Pd--Fe--O (nominal composition; prepared as inExample 1) to convert the CO in an exhaust gas was tested at lowtemperature (100° C.). A sample (1 g) of the dry material was exposed toa simulated mixture of automotive exhaust gas, at an equivalence ratio(λ) of 0.98 (Table 2) and flow rate of 2 dm³ min⁻¹. The conversion of COwas 95%, and showed no signs of declining with repeated testing(amounting to a total of 20 hours' use).

                  TABLE 2    ______________________________________    Composition of simulated exhaust gas mixture at λ = 0.98.    Component    Concentration/mol %    ______________________________________    H.sub.2      0.43    O.sub.2      0.72    CO           1.30    CH.sub.4     0.067    C.sub.3 H.sub.8                 0.0223    C.sub.3 H.sub.6                 0.0223    CO.sub.2     15.0    H.sub.2 O    9.2    NO           0.15    SO.sub.2     0.002    N.sub.2      balance    ______________________________________

EXAMPLE 4

Samples of 20% Pd--Fe--O (prepared as in Example 1) were aged under avariety of conditions before being re-tested under the exhaust gasmixture (at λ=0.98; Table 2). The ageing conditions and the activityresults are summarised in Table 3.

                  TABLE 3    ______________________________________    Effect of catalyst ageing on CO activity under an exhaust gas at    100° C.    Ageing conditions                    Temper-  Duration,                                    H.sub.2                                          CO conversion    SO.sub.2            H.sub.2 O                    ature, °C.                             hour   or O.sub.2                                          % at λ = 0.98    ______________________________________    (a) 0.002%  10%     400    0.5    1% H.sub.2                                             0    (b) 0.002%  10%     200    5.0    1% O.sub.2                                            82    (c) 0.002%  absent  400    0.5    1% H.sub.2                                            55    (d) absent  10%     200    0.5    1% H.sub.2                                            86    (e) 0.002%  absent  200    5.0    1% H.sub.2                                            35    (f) absent  absent  400    5.0    1% H.sub.2                                             0    (g) absent  10%     400    5.0    1% O.sub.2                                            75    (h) absent  absent  200    0.5    1% O.sub.2                                            93    ______________________________________

Complete loss of low-temperature activity occurred only when thematerial was aged under a reducing gas at high temperature (ie samples(a) and (f) in Table 3).

EXAMPLE 5

The ability of 20% Pd--Fe--O (prepared as in Example 1) to catalyse thewater-gas shift reaction at low CO concentration (in the presence of alarge excess of H₂ O) was measured over the temperature range 100°-200°C. A sample (2 g) was tested in a spinning-basket reactor (2500 rpm),using a gas mixture containing CO/H₂ O (1/100) in nitrogen at a flowrate of 2.5 dm³ min⁻¹. The results are shown in Table 4.

                  TABLE 4    ______________________________________    Steady-state activity data for water-gas shift reaction.    (Inlet and outlet concentrations/mol ppm)              Temperature    Catalyst  °C.                        Inlet (CO)                                 Outlet (CO)                                         Outlet (H.sub.2)    ______________________________________    20% Pd--Fe--O              100       998      830     112              110       1002     818     154              120       998      787     186              150       998      706     251              200       1000     650     340    α-Fe.sub.2 O.sub.3              100       1010     998      0              150       999      995      0              250       998      979      0    Fe--O*    250       1006     978      0    ______________________________________     *mildly reduced α-Fe.sub.2 O.sub.3 (0.5% H.sub.2 ; 360° C.;     20 min)

The difference between the rate of CO conversion and the rate of H₂release, at temperatures between 100° and 150° C., suggests that 20%Pd--Fe--O was being reduced by the hydrogen being generated. At 200° C.,this in situ reduction appeared complete, and the two rates becamealmost identical. The commercial sample of α-Fe₂ O₃ showed negligible H₂formation under identical conditions, even after mild reduction.

EXAMPLE 6

In order to determine the extent to which water-gas shift can contributewhen 20% Pd--Fe--O (prepared as in Example 1) is exposed to an exhaustgas, the CO conversion was measured both in the presence and absence ofO₂ in the simulated exhaust gas (see Table 2 for composition). At 100°C., the CO conversion dropped substantially (Table 5) when O₂ wasremoved from the simulated exhaust gas; the effect was much less at 165°C. The results show that direct oxidation of CO occurs at the lowesttemperatures, but the water-gas shift reaction begins to predominateabove 150° C.

                  TABLE 5    ______________________________________    Effect of O.sub.2 removal from exhaust gas on CO-conversion    over 20% Pd--Fe--O.              CO-conversion/%    Temperature °C.                Full exhaust gas                            Exhaust gas without O.sub.2    ______________________________________    100         95          32    140         99          43    165         99          77    ______________________________________

EXAMPLE 7

4% Pd--Fe--O (nominal composition) was prepared as described in Example1, except the mass of Pd in the nitrate precursor was 0.46 g. A sample(1 g) of the dry precipitate was tested under a gas mixture ofbut-1-ene/air (1/6) at a flow-rate of 100 cm³ min⁻¹. Unlike theperformance expected for catalysts derived from iron oxide (eg see Zhanget al, J. Chem. Soc. Faraday Trans., 88 (1992) 637), Pd--Fe--O showedsubstantial activity (including oxidative dehydrogenation) attemperatures below 200° C. (Table 6).

                  TABLE 6    ______________________________________    Butene oxidation activity of fresh 4% Pd--Fe--O as a function of furnace    temperature; activity measured after 20 min on line.                    Selectivity %    Temperature               Trans  Cis    °C.            Conversion %                       CO.sub.2                              but-2-ene                                     but-2-ene                                            butadiene    ______________________________________    180     74         30.5   30     22     4.5    135     61         43     23     18     15     80     44.5       45     17.5   15     22    (molar conversions and selectivities)    ______________________________________

At a furnace temperature of 80° C., the catalyst bed temperature rose toca 130° C. during reaction. The heat generated was then sufficient tosustain the reaction without further heat input from the furnace.

The selectivity of Pd--Fe--O to butadiene improved as a function of timeon line (Table 7), and could be further enhanced by adjusting thebutene/air ratio in the gas feed (Table 7).

                  TABLE 7    ______________________________________    Enhancing oxidative dehydrogenation of butene over    4% Pd--Fe--O at 100° C.    (a) Effect of conditioning    (activity measured for butene/air = 1/6)                    Selectivity             Conversion       Trans  Cis             %         CO.sub.2                              but-1-ene                                     but-2-ene                                            butadiene    ______________________________________    2 min on line             65        45     14     12     15    5 h on line             54        36      3      2     59    * 2 min on line             70        42     13     11     21    ______________________________________    * after "regeneration" of the 5 h used catalyst under air at 500°    C.    (b) Effect of gas composition    (activity of sample aged on line for 5 h)                    Selectivity %                              Trans  Cis    Butene/Air            Conversion %                       CO.sub.2                              but-2-ene                                     but-2-ene                                            butadiene    ______________________________________    1/6     54         36     3      2      59    1/4     51         28     2      1      69    1/2     23         31     4      5      60    ______________________________________

Comparative Example 1

In order to assess the significance of the results presented in Example7, a number of related materials were prepared and tested:

(i) Impregnated 4% Pd--Fe--O was prepared by adding aqueous Pd(NO₃)₃(containing 0.153 g Pd) to FeO(OH) (3.73 g) to form a thick paste, whichwas heated gently on a hot plate. The warm paste was transferred to anoven (110° C.) for drying (16 h), before calcination (500° C.; air; 2h).

(ii) Precipitated Pd--O was prepared by adding aqueous Na₂ CO₃ toaqueous Pd(NO₃)₃ (containing 2.30 g Pd). The precipitate was isolated,washed and dried (110° C.; 16 h).

(iii) Precipitated Fe--O was prepared by adding aqueous Na₂ CO₃ toaqueous Fe(NO₃)₃.9H₂ O (52.2 g). The precipitate was isolated, washedand dried (110° C.; 16 h).

(iv) Precipitated 4% Au/Fe₂ O₃ was prepared by the method described inExample 2.

A sample (1 g) of each material was tested under a gas feed ofbut-1-ene/air (1/6) at a flow rate of 100cm³ min⁻¹ (Table 8). None ofthese materials was active at temperatures below 150° C. Between 150°and 200° C., precipitated Pd--O showed reasonable activity, but theselectivity to butadiene was very low. Over a similar temperature range,impregnated 4% Pd--Fe--O was more selective, but the activity was poor;4% Au/Fe₂ O₃ showed high initial activity, but this was not sustainable.Precipitated Fe--O needed to be used above 300° C. before the yield ofbutadiene was comparable to that of precipitated Pd--Fe--O at 80°-100°C.

                  TABLE 8    ______________________________________    Butene oxidation performance at minimum operating temperature    (ie, the minimum temperature at which measurable amounts of product are    formed) (T); activity measured after 20 min on line.                         Selectivity, %    Catalyst T °C.                     Conversion, %                                CO.sub.2                                     but-2-ene                                            butadiene    ______________________________________    Precipitated             150     42         54   43     2.5    Pd--O    Precipitated             300     41         51   14     35    Fe--O    Impregnated             150      5         30   38     31.5    4% Pd--Fe--O    Precipitated               150 * 70         38   30     20    Au/Fe.sub.2 O.sub.3    ______________________________________     * rapid deactivation occurs at this temperature

EXAMPLE 8

Pd--Bi--Mo--O, with a nominal Pd-loading of 10% (by mass) and Bi/Momolar ratio of 2/1, was prepared by co-precipitation. Initially, aqueousPd(NO₃)₃ (containing 0.45 g Pd) was added to a solution of Bi(NO₃)₃.5H₂O (6.06 g) dissolved in 30% HNO₃ (20 cm³). Ammonium molybdate (7.86 gdissolved in 10% aqueous ammonia) was then added dropwise, with veryrapid stirring; some precipitation occurred during addition. The pH ofthe resultant suspension was adjusted to 7.4 (using concentrated aqueousammonia), completing the precipitation of a fine yellow powder. Theprecipitate was isolated, washed, dried (110° C.; 16 h) and calcined(500° C.; air; 4.5 h).

When a sample (1 g) of this material was tested under but-1-ene/air(1/7) at a flow-rate of 100 cm³ min⁻¹, the minimum operating temperaturewas ca 200° C. The yield of butadiene gradually increased over the firstfew minutes, before stabilising after 20-25 min (Table 9). This activitywas at a temperature 150°-200° C. lower than the minimum expected formixed-metal oxide catalysts (C. F. Cullis et al in "Catalysis", editedby G. C. Bond and G. Webb, Royal Society of Chemistry, London, 1982,page 273).

                  TABLE 9    ______________________________________    Butene oxidation activity of 10% Pd--Bi--Mo--O at 200° C.,    as a function of time on line.                       Selectivity, %    Elapsed time, min              Conversion, %                         CO.sub.2                                 but-2-ene                                         butadiene    ______________________________________     2        55         30.5    24.5    45    25        65         23      19.5    57.5    50        60         20      22      58    ______________________________________

Comparative Example 2

For comparison (to Example 8), an unmodified bismuth molybdate catalyst(in which the molar ratio of Bi/Mo=2/1) was prepared by a standard route(Ph Batist et al, J. Catal., 25 (1972), 1). Ground Bi(NO₃)₃.5H₂ O (6.06g) was added to concentrated aqueous ammonia (15 cm³), and stirred for 5min. The resultant suspension was filtered to isolate the white powder,which was then washed free of ammonia. The powder was added to H₂ MoO₄(1.03 g) in distilled water (150 cm³), and the mixture was heated underreflux (18 h). The solid product was isolated, dried (110° C.; 2 h) andcalcined (500° C.; 2 h).

A sample of the bismuth molybdate (1 g) was tested at 200° C., underbut-1-ene/air (1/7) at a flow-rate of 100 cm³ min⁻¹ (Table 10). Someinitial activity was observed, but only for the first few minutes.Sustainable activity was not achieved until the temperature was raisedto 350°-400° C., when the material functioned as a very selectivecatalyst for butadiene formation (Table 10b). On lowering thetemperature back down to 200° C., no activity was observed.

                  TABLE 10    ______________________________________    Butene oxidation activity of unmodified bismuth molybdate, as a    function of time on line.                       Selectivity, %    Elapsed time, min              Conversion, %                         CO.sub.2                                 but-2-ene                                         butadiene    ______________________________________    (a) 200° C.     2        25         3       61      36    25         3         1       44      55    50         0         --      --      --    (b) 350° C.     2        75         8       13.5    70    25        83         5.5     14      62.5    45        83         6.5     14      67.5    ______________________________________

EXAMPLE 9

Pd/CeO₂, with a nominal Pd-loading of 4% (by mass) was prepared byco-precipitation. A mixed solution was prepared by adding at ambienttemperature Ce(NO₃)₃.6H₂ O (126 g of solid) to aqueous Pd(NO₃)₃ (26.04 gof solution≡2.0 g Pd). This solution was added dropwise to a boilingsolution of NaOH (37.08 g) dissolved in the minimum amount of distilledwater required to dissolve it. The resultant suspension was maintainedat 100° C. for 1.25 hours. The precipitate was then isolated (byfiltration), washed, dried (110° C.; 16 h) and calcined (700° C.; air; 2h). Elemental analysis of the material showed it to contain 3.87% Pd(and <0.01% Na) by mass.

When a sample (1 g) was tested under either isobutane/N₂ (1/5) orisobutane/air (1/2) at a flow-rate of 100 cm³ min⁻¹, optimum activityfor the formation of isobutene occurred at 400° C. (Table 11). The Tablealso shows that the short-term yield of isobutene could be improved byusing a physical mixture of the Pd/CeO₂ (0.6 g) and Sb₂ O₄ (0.4 g).

                  TABLE 11    ______________________________________    Conversion of isobutane at 400° C., as a function of time on    ______________________________________    line    (a) Direct dehydrogenation (under isobutane/N.sub.2)                                      Isobutene    Catalyst Elapsed Time, min                          Conversion, %                                      selectivity %    ______________________________________    4% Pd/CeO.sub.2              2           7.5         >98             20           7           >98             35           5.5         >98    4% Pd/CeO.sub.2 +              2           8.5         >98    Sb.sub.2 O.sub.4             20           5           >98    ______________________________________    (b) Oxidative dehydrogenation (under isobutane/air)                           Selectivity, %    Catalyst Elapsed Time, min                          Conversion, %                                     *CO.sub.x                                          Isobutene    ______________________________________    4% Pd/CeO.sub.2              2           10         61   39             20           10         58   42    4% Pd/CeO.sub.2 +              2           15         45   55    Sb.sub.2 O.sub.4             20            7         80   20    ______________________________________     *CO.sub.x = CO.sub.2 + CO

Comparative Example 3

For comparison (to Example 9), a physical mixture of CeO₂ (0.6 g) andSb₂ O₄ (0.4 g) was tested under isobutane/N₂ (1/5) and isobutane/air(1/2), at flow-rates of 100 cm³ min⁻¹ (Table 12). The yields ofisobutene were much lower than for either 4% Pd/CeO₂ or 4% Pd/CeO₂ +Sb₂O₄, with negligible activity under isobutane/N₂ at temperatures below500° C.

                  TABLE 12    ______________________________________    Isobutane dehydrogenation activity of CeO.sub.2 + Sb.sub.2 O.sub.4    ______________________________________    (a) Direct dehydrogenation (under isobutane/N.sub.2)    Temperature, °C.                 Maximum isobutane conversion, %    ______________________________________    400          0    500          1    550          1.5    ______________________________________    (b) Oxidative dehydrogenation (under isobutane/air) at 400° C.                        Selectivity %    Elapsed time, min                Conversion %  CO.sub.x                                     Isobutene    ______________________________________     2          7             76     11    20          6             73     15    ______________________________________

We claim:
 1. A method of conducting a chemical reaction, which methodcomprises contacting a reactant with a catalyst composed of metal oxideparticles among which are uniformly incorporated, in order to reduce theoperating temperature of the catalyst, palladium particles, wherein thereaction is the oxidative dehydrogenation of but- 1-ene to butadiene,and wherein the reaction is conducted at a temperature below 200° C. andthe metal oxide comprises iron (III) oxide.
 2. A method according toclaim 1, wherein the catalyst contains 0.1-25% by mass of the palladiumparticles based on the total mass of the palladium particles and themetal oxide particles.
 3. A method according to claim 1, wherein thecatalyst contains antimony oxide as additive.
 4. A method according toclaim 1 wherein the catalyst is employed without prior calcination toactivate it.
 5. A method according to claim 1, wherein the reaction isself-sustaining without the further supply of heat once the reaction hasstarted.
 6. A method according to claim 1, wherein the catalyst is atleast partially amorphous, and has a catalyst surface having metal ionsand Pd²⁺ ions on the catalyst surface.
 7. A method according to claim 1,wherein the catalyst comprises a catalyst particle comprising metaloxide particles and palladium particles, wherein the palladium particlesare uniformly incorporated among the metal oxide particles, in order toreduce the operating temperature of the catalyst.
 8. A method accordingto claim 1, wherein the catalyst particles respectively have a diameterof less than 150 microns.
 9. A method according to claim 1, wherein thecatalyst is prepared by co-precipitation of the metal oxide particlesand the palladium particles.
 10. A method of conducting a chemicalreaction, which method comprises contacting a reactant with a catalystcomposed of metal oxide particles among which are uniformlyincorporated, in order to reduce the operating temperature of thecatalyst, palladium particles, wherein the reaction is the water-gasshift reaction and the metal oxide comprises iron(III) oxide, andwherein the reaction is conducted at a temperature of 100°-200° C.
 11. Amethod according to claim 10, wherein the catalyst contains 0.1-25% bymass of the palladium particles based on the total mass of the palladiumparticles and the metal oxide particles.
 12. A method according to claim10, wherein the catalyst contains antimony oxide as additive.
 13. Amethod according to claim 10, wherein the catalyst is employed withoutprior calcination to activate it.
 14. A method according to claim 10,wherein the catalyst is at least partially amorphous, and has a catalystsurface having metal ions and Pd²⁺ ions on the catalyst surface.
 15. Amethod according to claim 10, wherein the catalyst comprises a catalystparticle comprising metal oxide particles and palladium particles,wherein the palladium particles are uniformly incorporated among themetal oxide particles, in order to reduce the operating temperature ofthe catalyst.
 16. A method according to claim 10, wherein the catalystparticles respectively have a diameter of less than 150 microns.
 17. Amethod according to claim 10, wherein the catalyst is prepared byco-precipitation of the metal oxide particles and the palladiumparticles.